In the field of wireless communication system, one of the most common interferences is occurred from a transmitter output modulation signal amplified by a power amplifier, then, an electromagnetic radiation or a substrate coupling phenomenon may interfere in the local oscillation signal to lead a deterioration of spectral purity and phase noise therefore severely effecting the output quality of the communication system.
Referring to FIG. 9, a conventional direct-conversion transmitter 200 comprises a signal amplifier 210, a local oscillation circuit 220, a radio-frequency mixer 230 electrically connected with the signal amplifier 210 and the local oscillation circuit 220, a quadrature modulator 240 electrically connected with the radio-frequency mixer 230 and a power amplifier 250 electrically connected with the quadrature modulator 240. The direct-conversion transmitter 200 generates the required modulation RF transmit signal. However, owing to the nature of oscillators, local oscillation circuit 220 is subject to the external interference, which often causes frequency pulling effects to contaminate the spectral purity, and thus transmit signal quality. Additionally, there always exists signal distortion due to non-ideal physical characteristics of conversion circuit. For the reason that the modulation signal of the direct-conversion transmitter passes through the radio-frequency mixer 230, mixes with distorted local oscillation signal and amplifies via the power amplifier 250, a critical deterioration of the communication quality for the direct conversion transmitter 200 is occurred. Therefore, designer should improve the anti-interference capability of local oscillator and the linearity to achieve the predetermined signal quality specification of general wireless communication system.